The present invention relates, in one aspect, to yttria sol used as a binder for a wide variety of refractory materials, casting molds, core molds, related compositions and processes, and casting of reactive metals into ceramic molds. More particularly, the invention pertains to the use of yttria sol as a bonding agent for refractory materials in making the first coat of a ceramic mold for casting of reactive metals such as titanium and titanium alloys. Backup coating compositions may be any of the compatible system known in the art.
Molds produced in accordance with the present invention are particularly useful because they enable the casting of reactive metals with minimized or essentially no alpha case. The present invention results in lower alpha case than is obtainable with conventional molds utilizing other binders previously used in the art. Methods of forming casting molds and core molds are encompassed by the present invention.
In another aspect, the present invention pertains to the use of zirconia sol with selected refractory materials; namely, fused yttrium oxide, or mixtures of fused yttrium oxide and fused zirconium oxide, and the formation of coating compositions therewith and methods of forming molds utilizing same.
For the past 25 years, considerable effort has been devoted to providing capability for casting reactive metals, particularly titanium and its alloys, into ceramic molds. This development and the interest in providing such capability resulted from the interest and activity in the nuclear and aircraft industries where it was necessary to search for high strength and lightweight metals. Since the strength to weight ratio for titanium is very high, use in the aircraft industry was a logical development.
The melting point of titanium metal is almost 3100.degree. F. and reacts in the molten condition with most refractories. Earlier attempts to cast titanium into ordinary foundry molds were unsuccessful due to the undesirable chemical reactions between the hot metal and the surfaces with which it came into contact. For example, reduction of the silica produced heavy reaction zones on the casting surface and oxide inclusions. This reaction layer is known in the industry as "alpha case". Problems of alpha case have been described in detail in the literature. Machined and formed graphite molds have been used commercially to make titanium castings. Such molds can be made in such a way as to minimize the alpha case layer. The U.S. Bureau of Mines activities have continued for almost 25 years on the casting of refractory metals. However, there have been continuing efforts to search for other materials and methods to reduce or eliminate alpha case.
The use of graphite in investment molds has been described in the art in such patents as U.S. Pat. Nos. 3,241,200; 3,243,733; 3,256,574; 3,266,106; 3,296,666 and 3,321,005 all to Lirones. Other prior art which show a carbonaceous mold surface utilizing graphite powders and finely divided inorganic powders called "stuccos" are Operhall, U.S. Pat. No. 3,257,692; Zusman et al., U.S. Pat. No. 3,485,288 and Morozov et al., U.S. Pat. No. 3,389,743. These documents describe various ways of obtaining a carbonaceous mold surface by incorporating graphite powders and stuccos, various organic and inorganic binder systems such as colloidal silica, colloidal graphite, synthetic resin which are intended to reduce to carbon during burnout, and carbon coated refractory mold surfaces. These systems are observed to have the disadvantage of the necessity for eliminating oxygen during burnout, a limitation on the mold temperature and a titanium carbon reaction zone formed on the casting surface.
Further developments including variations in foundry molds are shown in Turner et al., U.S. Pat. No. 3,802,902 which uses sodium silicate bonded graphite and/or olivine which was then coated with a relatively non-reactive coating such as alumina. However, this system still did not produce a casting surface free of contamination.
Schneider, U.S. Pat. No. 3,815,658 molds which are less reactive to steels and steel alloys containing high chromium, titanium and aluminum contents in which a mangnesium oxide-forsterite composition is used as the mold surface.
A number of attempts have been made in the past to coat the graphite and the ceramic molds with materials which would not react with the reactive metals being casted. For example, metallic powders such as tantalum, molybdenum, columbium, tungsten, and also thorium oxide had been used as non-reactive mold surfaces with some type of oxide bond. See Brown, U.S. Pat. Nos. 3,422,880; 3,537,949 and 3,994,346.
Operhall, U.S. Pat. No. 2,806,271 shows coating a pattern material with a continuous layer of the metal to be cast, backed up with a high heat conductivity metal layer and investing in mold material.
Basche, U.S. Pat. No. 4,135,030 shows impregnation of a standard ceramic shell mold with a tungsten compound and firing in a reducing atmosphere such as hydrogen to convert the tungsten compound to metallic tungsten or tungsten oxides. These molds are said to be less reactive to molten titanium but they still have the oxide problems associated with them.
Brown, U.S. Pat. No. 4,057,433 discloses the use of fluorides and oxyfluorides of the metals of Group IIIa and the lanthanide and actinide series of Group IIIb of the Periodic Chart as constituents of the mold surface to minimize reaction with molten titanium. This reference also shows incorporation of metal particles of one or more refractory metal powders as a heat sink material. However, even those procedures have resulted in some alpha case problems.
A development by General Electric has provided barrier layers of refractory oxide in a silica bonded mold for casting alloys containing significant amounts of reactive metals; see Gigliotti et al. U.S. Pat. Nos. 3,955,616; 3,972,367 and 4,031,945.
Huseby, U.S. Pat. No. 4,240,828 shows casting a super alloy (containing nickel and cobalt) material containing reactive metals into a ceramic mold containing a rare earth dopant in the alloy.
In the 1960's, developments at Wright Air Development Center led to the formation of a crucible for melting titanium formed from a titanium enriched zirconium oxide crucible with less reaction to molten titanium than pure zirconium oxide.
Richerson, U.S. Pat. No. 4,040,845 shows a ceramic composition for crucibles and molds containing a major amount of yttrium oxide and a minor amount of a heavy rare earth mixed oxide. Such methods including the making of a titanium metal enriched yttrium oxide were only partially successful because of the elaborate expensive and technique which required repetitive steps.
Molds for casting molybdenum made from zirconium acetate bonded calcia stabilized zirconium oxide have been made by the Bureau of Mines. p Feagin, U.S. Pat. No. 4,415,673 has reported on the zirconia binder which is an aqueous acidic zirconia sol used as a bond for an active refractory including stabilized zirconia oxide thereby causing reaction and gelation of the sols. Solid molds were made for casting depleted uranium. A distinction is made in this patent between "active" refractories and refractories which are relatively inert. The compositions of Feagin are intended to contain at least a portion of active refractories. See also Feagin, U.S. Pat. 4,504,591.
In summary, the following are four of the processes used for the production of molds for titanium casting and can be set forth as follows:
1. Graphite mold coated with a tungsten powder and a silica binder.
2. A silica binder prepared either from ethyl silicate, aqueous colloidal silica or the like, used to bond thorium oxide for a first coating on a wax pattern. This is then backed up with silica bonded lower cost refractory coatings and stuccos to complete the mold.
3. A zirconium acetate bonded zirconia refractory is used for a first coating. Setting or gelling taken place immediatedly after application in an atmosphere of ammonia. After drying, this is backed up with a lower cost silica bonded refractory coating to complete the shell mold.
4. A colloidal silica bonded stabilized zirconium oxide (ZrO.sub.2) refractory is used to form the first coat of a mold and this is backed up with a lower cost silica bonded refractory coating to complete the shell mold.
It is generally recognized in the industry that all commercial processes have some alpha case on their casting. This may range from about 0.005 inches to 0.04 inches in thickness depending on process and casting size. The alpha case must be milled off by chemical means or other means from the casting before a satisfactory casting is obtained. The extra cost imposed by the chemical milling operation is a disadvantage and presents a serious problem from the standpoint of accuracy of dimensions. Normally, the tooling must take into consideration the chemical milling which results in the removal of some of the material in order to produce a casting that is dimensionally correct. However, since casting conditions vary, the alpha case will vary along the surface of the casting. This means that there is a considerable problem with regard to dimensional variation.